Most of aromatic hydrocarbons which are useful as chemical industrial raw materials, such as benzene, toluene and xylene, have been produced in the past as by-products in gasoline production in the petroleum refining industry or ethylene production in the petrochemical industry. In either case, such aromatic hydrocarbons are not the desired products, and therefore, yields on the basis of crude oil that is a starting raw material are not high. Further, the production is controlled by the supply-demand situation on the side of each desired product. As a production process for an aromatic hydrocarbon that is a desired product, a process using a light component derived from crude oil has been developed, and a part of it has been commercialized, but the production thereof still remains small.
On the other hand, the amounts of natural gas reserves in the whole world are said to be about 6600 TCF (1 TCF is an abbreviation of one trillion cubic feet), but most of them have not been used effectively. The technique to produce an aromatic hydrocarbon from methane that is a main component of natural gas is a method capable of not only adding a high value to abundant natural gas but also converting raw material sources of aromatic hydrocarbons that are important chemical industrial raw materials from crude oil resources into non-crude oil resources, and practical use thereof has been desired.
As a catalyst which is widely known to exhibit excellent performance as a catalyst capable of directly producing an aromatic hydrocarbon using methane as a raw material and which has been best studied, a zeolite-supported molybdenum catalyst (non patent literature 1) found by L. Wang, et al. in 1993 can be mentioned. In techniques having been disclosed so far, crystalline metallosilicate having a transition metal supported thereon, particularly, MFI-type zeolite or MWW-type zeolite having molybdenum, tungsten or rhenium supported thereon, is widely known as a catalyst capable of directly producing an aromatic hydrocarbon from methane efficiently.
As the temperature becomes higher in the reaction to produce an aromatic hydrocarbon from methane, the reaction becomes more advantageous because of thermodynamic equilibrium. For example, in the reaction to produce benzene from methane, the equilibrium conversion in the reaction at 700° C. is about 11%, while the equilibrium conversion in the reaction at 800° C. is estimated to be about 20%. In order to efficiently produce an aromatic hydrocarbon, therefore, the reaction temperature of this reaction system is limited to 600° C. or higher, preferably 700° C. or higher.
Moreover, it is known that a carbonaceous substance is deposited on the above catalyst during the reaction and causes deactivation of the catalyst. The carbonaceous substance deposited on the catalyst is burned off in a high-temperature oxygen-containing atmosphere. Then, in order to use the catalyst for a long period of time, a method of alternately repeating a reaction step and a catalyst regeneration step of heat-treating the catalyst in an oxygen-containing atmosphere has been proposed.
However, if the reaction step and the regeneration step are alternately repeated actually, the catalyst is gradually deteriorated, and production of an aromatic compound cannot be carried out over a long period of time, so that this method has not been put to practical use yet (e.g., non patent literature 2).
The cause of deterioration of the catalyst has not been clarified sufficiently, but some estimated mechanisms described below have been proposed. That is to say, a theory that a part of a crystalline structure of a crystalline metallosilicate is thermally collapsed under the high-temperature conditions to thereby exhibit lower catalytic performance (non patent literature 3), a theory that the melting point of molybdenum oxide is as low as 795° C., and therefore, if catalyst regeneration treatment in an oxygen-containing atmosphere is carried out at a high temperature, decrease in the number of active sites due to vaporization or sintering is brought about (non patent literature 4), a theory that at a high temperature, molybdenum partially reacts with an aluminum atom in a metallosilicate crystal lattice to form an inert Al2(MoO4)3 species, and this leads to decrease in the number of active sites (non patent literatures 5, 6 and 7), etc. can be mentioned.
There is a possibility that the catalyst deterioration is brought about by exposure of the catalyst containing molybdenum oxide to a high temperature whatever the deterioration mechanism may be, and therefore, it is thought that if the treatment temperature can be lowered in the catalyst regeneration step, the deterioration can be inhibited. However, if the treatment temperature in the catalyst regeneration step is low, removal of the deposited carbonaceous substance by burning becomes insufficient, and the catalytic activity cannot be completely recovered. On that account, the deposited carbonaceous substance is removed with inhibiting catalyst deterioration in the catalyst regeneration step, and therefore, the catalyst regeneration temperature is limited. For example, in a patent literature 1, a temperature of 400 to 500° C. is given as an example of the catalyst regeneration temperature.
In order to efficiently remove the carbonaceous substance in the catalyst regeneration step, a method of combining a carbonaceous substance removing treatment using a reducing gas such as hydrogen with a carbonaceous substance removing treatment using an oxidizing gas has been proposed in, for example, a patent literature 3. In a non patent literature 8, in order to reduce catalyst deterioration in the regeneration step for a catalyst, an attempt to lower the regeneration temperature has been made by adding a small amount of nitrogen monoxide to air that is a regeneration gas. In the non patent literature 8, it is described that as compared with a case where a catalyst is regenerated in air at 550° C., the regeneration temperature can be lowered down to 450° C. in the case where a mixed gas of air and nitrogen monoxide is used, and as a result, the number of regeneration times of the catalyst can be increased. However, it is said that even if a regeneration method using air to which a small amount of nitrogen monoxide has been added is used, the catalytic activity is gradually lost by repeating the reaction step and the regeneration step, and therefore, a more efficient catalyst regeneration method has been desired by the industry.
From the above, improvement in thermal stability (durability) of a catalyst is an industrial problem from the viewpoint of the catalyst life. With regard to a catalyst for producing an aromatic hydrocarbon from a hydrocarbon containing methane as a main component, it has been found in, for example, a patent literature 2 that thermal stability of a crystalline metallosilicate can be improved by combining inhibition of elimination of a metal from the crystalline metallosilicate with a surface modification treatment using a transition metal or an alkaline earth metal. However, a specific technique to inhibit lowering of activity accompanying repetition of the reaction step and the regeneration step has not been disclosed at all.
Moreover, several techniques to improve catalytic performance by adding a second metal component have been disclosed so far. For example, in a patent literature 4, performance of a catalyst constituted of at least one metal selected from Mo, Ce and Cs, La and zeolite is disclosed. In a patent literature 5, performance of a catalyst constituted of Mo, a transition metal (at least one metal selected from Ti, Zr, Cr, W, Co, Ru and Ni), a rare earth metal (at least one metal selected from La, Ce, Pr, Nd and Sm) and zeolite is disclosed. In a patent literature 6, a preparation process for a methane dehydroaromatization catalyst, which is characterized by comprising a step of heating a catalyst precursor containing molybdenum and aluminosilicate in the presence of a treating gas containing propane in order to improve efficiency of the methane dehydroaromatization catalyst, is disclosed, and it is described that the catalyst precursor preferably contains a metal (Ga, Zn, Nb, Zr, La, Co, Fe, Ce, Ag, Y, V, Sr, W, Yb, Sm, Ni, Ru, Rh, Pt, Cu, Au, Al, Ti, Pb, Re, Ir, Si, Sn and Pd) as a promoter. That is to say, this process relates to an invention in which a catalyst is changed to be in such a state that it can maintain a high yield for a long time by a technique of, for example, preliminarily contacting the catalyst with a certain kind of a gas prior to the reaction, but description of heat resistance (thermal stability) of a molybdenum-based supported catalyst, namely, description of a method for efficiently regenerating a catalyst whose activity has been lowered, by removing carbon produced on the catalyst as a by-product at a high temperature during aromatization reaction of a hydrocarbon such as methane, is not observed.
Further, it is disclosed that the chemical properties of Mo are changed by the addition of a second metal (e.g., non patent literature 9, non patent literature 10), but it cannot be said yet that the stability of catalytic activity is satisfactory.